Among electrodeposition coating methods, matte electrodeposition coating can provide a minimized metallic luster and thus produce a reposed atmosphere. In recent years, the matte electrodeposition coating has rapidly found a wide application to the surface protection as well as the improvement of the appearance of various metallic parts. Various matte electrodeposition coating methods for acrylic resin have been proposed.
The inventors of the present invention previously found that the mixing of a premix of an aluminum complex compound (hereinafter occasionally referred to as "complex compound") and an amino resin with a solution of an acrylic resin in an organic solvent can provide a matte electrodepositable coating that can be stably applied without causing agglomeration or precipitation of particles and thickening or gelation of the coating bath prepared from the matte electrodeposition coating, as described in JP-A-3-207773. The term "JP-A" as used herein means an unexamined published Japanese patent application.
However, the weather resistance of acrylic electrodeposition coating films for use in aluminum building materials leaves much to be desired when applied to the external wall of multistory buildings requiring a high weather resistance over a prolonged period of times as several decades, though showing no problems when applied to ordinary houses.
In order to eliminate these problems, matte electrodeposition coating compositions comprising a fluorine-containing resin having a good weather resistance as a constituent of a coating film have been proposed. For example, JP-A-3-39369 discloses a coating composition comprising a base resin and a crosslinking agent as essential components, wherein the base resin is a mixture of a resin (A) and a resin (B); at least one of the resins (A) and (B) is a fluorine-containing resin; the acid value of the resins (A) and (B) are 40 to 150 and less than that of the resin (A), respectively; and the difference in acid value between the two resins being not less than 40. The coating composition can allegedly form a matte coating film with a good durability and appearance.
However, the above prior art technique, though capable of forming a matte coating film, is disadvantageous in that the use of a fluorine-containing resin and an acrylic resin which greatly differ from each other in acid value (although it is stipulated as not less than 40 in the above JP-A-3-39369, its preferable value is described as not less than 60 in its detailed description and 90 in Examples thereof) provides a poor compatibility therebetween, reducing the transparency of the coating film. In addition, a fluorine-containing resin is disadvantageous in that it has a low acid value and hydroxyl value and thus provides an electrodepositable coating which exhibits too low a throwing power to be uniformly applied to a material to be coated.